Definition: Star clusters are large groups of stars held together by self-gravitation.
Two main types of star clusters can be distinguished: globular clusters are tight groups of ten thousand to millions of old stars which are gravitationally bound,
while open clusters are more loosely clustered groups of stars, generally containing fewer than a few hundred members, and are often very young.
Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the galaxy,
but cluster members will continue to move in broadly the same direction through space even though they are no longer gravitationally bound;
they are then known as a stellar association, sometimes also referred to as a moving group.
Star Cluster Types: There are two types of star clusters, Globular Star Clusters and Open Star Clusters.
Open Clusters: Open cluster is a group of up to a few thousand young stars that were formed from the same giant cloud and have roughly the same age.
Open clusters are very different from globular clusters. Unlike the spherically distributed globulars, they are confined to the galactic plane, and are almost always found within spiral arms.
They are generally young objects, up to a few tens of millions of years old, with a few rare exceptions as old as a few billion years,
such as Messier 67 (the closest and most observed old open cluster) for example.
They form H II regions such as the Orion Nebula.Open clusters typically have a few hundred members and are located in an area up to 30 light-years across.
Being much less densely populated than globular clusters, they are much less tightly gravitationally bound, and over time, are disrupted by the gravity of giant molecular clouds and other clusters.
Close encounters between cluster members can also result in the ejection of stars, a process known as "evaporation". The Milky Way contains over 1,000 open clusters.Many open clusters are very loosely organized and difficult to identify.
Some open clusters are more organized and therefore easier to identify. Several open clusters are visible to the unaided eye. Many more are visible with binocular or small telescope.
Globular Clusters: Globular clusters are roughly spherical groupings of from 10 thousand to several million stars packed into regions of from 10 to 30 light-years across.
They commonly consist of very old Population II stars – just a few hundred million years younger than the universe itself –
which are mostly yellow and red, with masses less than two solar masses.
Such stars predominate within clusters because hotter and more massive stars have exploded as supernovae, or evolved through planetary nebula phases to end as white dwarfs.
Yet a few rare blue stars exist in globulars, thought to be formed by stellar mergers in their dense inner regions; these stars are known as blue stragglers.Globular cluster is a spherical collection of stars that are all gravitationally bound.
Globular clusters are fairly common, there are about 150 globular clusters in our Milky Way. Globular clusters are composed of hundreds of thousands of low metal, old stars. Globular clusters have a high density of stars.
There are a few globular clusters visible by naked eye, and several visible with binocular or small telescope.